She began acting at age seven in local theater productions and played the title role in Annie (2012–14) and young Elizabeth II in The Audience (2015) on Broadway.

Sink had her breakthrough portraying Max Mayfield in the Netflix science fiction series Stranger Things (2017–present) and received critical acclaim for her performance in its fourth season.

Sink has since starred in Darren Aronofsky's psychological drama The Whale (2022), for which she received a Critics' Choice Movie Award nomination.

[14][15] Sink was regularly performing in plays at Theater Under the Stars by the age of nine;[3] she appeared in musical productions of White Christmas (2011) and portrayed the title role in Annie (2012).

[18] Following the departure of Lilla Crawford at the end of July, Sink and Taylor Richardson began alternating between the roles of Annie and Duffy.

[15] During her Annie Broadway run, Sink made her television debut in 2013 in a guest role on the spy drama series The Americans.

[22] In 2015, Sink starred as Suzanne Ballard in the NBC action thriller series American Odyssey,[23] which was canceled after one season.

[24] That year, Sink appeared in the Broadway production of The Audience as young Queen Elizabeth II, who is portrayed by Helen Mirren as an adult.

[28] In September 2016, Sink auditioned to play Maxine "Max" Mayfield in the second season of Netflix's science fiction drama series Stranger Things.

[30] According to director and writer Matt Duffer, casting Sink was "a bit of a no-brainer" due to her "innocent, child chemistry" with Matarazzo and McLaughlin.

[38] Sink, along with her Stranger Things cast members, was nominated for the SAG Award for Outstanding Performance by an Ensemble in a Drama Series.

[44][45][46] The BBC considered her performance "wonderfully loose and natural",[47] while Variety praised her and co-star Millie Bobby Brown's energy.

[52] Sink's acting received critical praise;[53] the Los Angeles Times commended her portrayal of Ziggy's emotions, attractions, and loyalties,[54] and RogerEbert.com said that her "intense performance gets a great deal of volume" from a one-dimensional character.

[56] Sink had a leading role opposite Dylan O'Brien in All Too Well: The Short Film (2021), which was written and directed by American musician Taylor Swift.

[64] Collider stated that Sink and O'Brien gave "vividly emotional performances" and told "an incredibly moving tale of love, power, gaslighting, and heartache".

[75] In February 2021, Sink was cast in the psychological drama The Whale following a Zoom meeting with director Darren Aronofsky and lead actor Brendan Fraser.

'"[14] Sink said filming could be "so draining" at times due to the darker subject matter requiring her to "fully shed every layer and be really vulnerable".

[77] Variety's Owen Gleiberman argued that Sink "acts with a fire and directness that recalls the young Lindsay Lohan",[78] while the Los Angeles Times's Justin Chang found her emotional intensity "impressive" but felt her role was poorly written.

[85][86] IndieWire praised her and co-star Eric Bana's performances, stating they "make for a pleasant viewing experience even when the [film's] intellectualism comes up short".

[90] That same year, she was included on the Time 100 Next; her profile was penned by her Stranger Things co-star Winona Ryder, who described Sink as "this creative acrobat and she's on this balance beam that very few have the courage to walk ... As an actor, she knows that we are ultimately in service to the characters and story".